Many different types of composite materials have been developed for different applications. One composite material which has achieved a substantial amount of attention for high temperature/durability applications is the combination of an integral ceramic article within a mass of metal.
The most practical and inexpensive method for forming a composite body having an integral ceramic surrounded by a mass of metal entails solidifying a cast molten metal around a ceramic article. However, when the cast metal solidifies and cools, high compressive stresses can occur in the ceramic article. Particularly, the thermal expansion coefficients of the ceramic and the metal typically differ from each other such that the stresses which are exerted upon the ceramic article can result in crack initiation and/or catastrophic failure of the ceramic. Such crack initiation and/or failure has been especially pronounced in low strength, hollow, ceramic articles. To date, there has not been an adequate resolution to this problem of excessive compressive stresses which can lead to the failure of low strength ceramic articles. Moreover, crack initiation and/or failure in a metal has also been a problem in certain applications. For example, when the metal surrounding the ceramic is thin, the greater magnitude of contraction of the metal during cooling can result in tensile stresses in the metal which can lead to yielding or failure thereof.
It frequently is desirable to form an integral ceramic article within a mass of metal for applications which require the conservation of exhaust gas thermal energy, for example, as an exhaust port for an internal combustion engine. Specifically, in reference to the exhaust port for an internal combustion engine, an integral, low strength, hollow ceramic article is at least partially surrounded by a mass of solidified metal. The thermal insulating properties of the ceramic will assist in heating-up a downstream catalytic converter substrate by the hot exhaust gasses at a rapid rate relative to an all-metal exhaust port because the ceramic reduces heat losses of the exhaust gas stream. Such rapid heating is desirable because a catalytic converter substrate does not convert undesirable pollutants from an exhaust gas until it has been heated-up to its operating temperature by the exhaust gas. Particularly, unacceptable amounts of pollutants may be discharged from an exhaust system during the initial warm-up period for a catalytic converter relative to the amount of pollutants discharged once the catalytic converter has been heated. Thus, by incorporating a ceramic within a metal exhaust port, undesirable emissions from an internal combustion engine can be reduced. Additionally, the use of a port liner in a turbocharger will result in higher exhaust stream temperatures, thereby improving the operating efficiency of the turbocharger. Moreover, use of a ceramic within a metal exhaust port reduces the heat energy input from the exhaust gas into the engine coolant. Thus, a smaller cooling system could be utilized for an internal combustion engine.
Various attempts have been made to reduce the compressive stresses on a ceramic article induced by the solidification and cooling of a molten metal around the ceramic. For example, U.S. Pat. No. 3,709,772 to Rice (hereinafter "Rice '772") discloses that a porous, fibrous, resilient refractory layer is applied to an outer surface of a hollow ceramic article prior to casting a metal around the ceramic. The porous refractory layer is applied by wrapping the outer surface of the ceramic article with an aluminum silicate fiber paper having a thickness of 0.120-0.200 inches. Alternatively, it is disclosed that the layer can be applied by "spraying" a liquid suspension of the aluminum silicate fibers against a surface or by "blowing" chopped fibers against a tacky surface. It is disclosed that a resilient layer within this thickness range can tolerate the stresses (i.e., prevent the ceramic from rupturing) generated by casting around the ceramic article an iron or an aluminum alloy having a thickness of 0.1875-0.250 inches.
However, the disclosed aluminum silicate layer is unacceptable in many instances such as sophisticated cylinder head castings, due to its thickness relative to the thickness of the surrounding metal layer. Thus, the strength of the composite body is compromised because of the relatively thick and weak intermediate resilient layer. Moreover, the aluminum silicate layer is wettable by certain molten metals such as aluminum and magnesium at typical metal casting temperatures. Thus, molten metal can tend to penetrate the aluminum silicate layer and thereby inhibit the resilient layer from functioning as desired.
U.S. Pat. No. 3,718,172 to Rice (hereinafter "Rice '172") discloses a cushioning layer of aluminum silicate which is similar to the resilient refractory layer disclosed in Rice '772. Accordingly, this layer suffers from all of the deficiencies of the resilient layer disclosed in Rice '772.
U.S. Pat. No. 4,245,611 to Mitchell et al. discloses the use of a cushioning layer of aluminum silicate between a ceramic insert and a metallic piston body. However, the disclosed cushioning material is similar to the cushioning material disclosed in each of Rice '772 and Rice '712. Thus, this cushioning material is undesirable for all of the reasons previously discussed.
Another alternative to suppressing undesirable stresses in a ceramic article involves applying a molding sand mixed with a binder on the ceramic article prior to casting molten metal around the ceramic. Japanese Patent No. 53-8326 discloses that a molding sand can be applied to an outer surface of the ceramic to form a covering layer having a 1-5 mm thickness thereon. The molding sand can be combined with water or a binder to assist in bonding the sand to the ceramic article. However, the sand is wettable by certain molten metals such as aluminum or magnesium at typical metal casting temperatures. Thus, the molten metal tends to penetrate the porous sand covering and inhibit the sand layer from cushioning against compressive stresses generated by the cooling metal.
In addition to providing some type of intermediate layer between a ceramic article and a solidified molten metal, focus has been placed upon controlling certain physical properties of the ceramic. Particularly, ceramic articles having a controlled porosity and pore size have also been utilized to ameliorate the effect of undesirable compressive stresses.
U.S. Pat. No. 3,568,723 to Sowards discloses casting a molten metal around a ceramic core, said core having a surface porosity in the range of 20-80% and a pore size in the range of 25-2500 microns. The surface porosity is achieved by modifying the surface of the ceramic core by such techniques as incorporating a decomposable material in a surface area of the core when the core is being made. This technique is cumbersome to control and adds expense to the process. Moreover, Sowards also discloses various coatings which can be applied on a surface of a ceramic core, said coatings including polystyrene, cemented sodium silicate, a quartz wool pad and silica frit.
U.S. Pat. No. 4,533,579 to Hashimoto also discloses using ceramic articles having specific physical properties. Hashimoto discloses that it is desirable to construct the ceramic to have a particular particle size distribution. Particles of less than 44 microns in size account for 14.5-50% of the total and the balance are particles with a maximum size ranging from 500-2000 microns. It is disclosed that this particle size distribution gives the ceramic an improved resistance to compressive stresses. However, failure of the ceramic is only part of the issue; providing some relaxation capability to avoid failure of the metal casting is also important and Hashimoto does not provide any means to avert this particular problem.
German Patent No. 2,354,254 controls the physical properties of a ceramic article to enhance the resistance of the ceramic to thermal stresses. It is disclosed that heat-insulated castings for exhaust ports of internal combustion engines are formed by casting metal around a flexible ceramic shell which has a smooth outer surface. It is necessary for the ceramic shell to have a modulus of elasticity of 200-5000 kg/mm.sup.2, a bending strength of 8-200 kg/cm.sup.2, and a wall thickness of less than one-fourth of its inside diameter.
Another ceramic article exhibiting particular physical properties is disclosed in U.S. Pat. No. 3,919,755 to Kaneko et al. This patent is directed to manufacturing heat insulating castings by molding a flexible porous ceramic liner from a mixture of a refractory material and an alumina cement; casting molten metal against the liner; and after casting the molten metal, impregnating the liner with a heat resistant binder. It is disclosed that it is important to avoid impregnating the ceramic liner with a heat resistant binder before casting molten metal to ensure that the liner will survive the casting process.
High strength ceramic articles have also been utilized to ameliorate the effect that thermal stresses have on the ceramic. For example, Japanese Patent Nos. 60-118366 and 60-216968 disclose casting the molten metal around high strength oxide ceramics. Finally, Japanese Patent No. 59-232978 discloses casting molten metal around high strength ceramic bodies of stabilized zirconia.
From the foregoing, it can be seen that previous attempts to ameliorate undesirable stresses involved using ceramic articles having relatively thick, porous coatings which are wettable by metals such as aluminum and magnesium at typical metal-casting temperatures; and using ceramics having carefully controlled physical properties. However, ceramic-metal composite bodies which employ thick coatings on a ceramic article are prone to physical damage due to the presence of a relatively thick and weak layer between the metal and the ceramic. Moreover, such coatings can be difficult, and in certain cases expensive, to apply. These known coatings have historically needed to be thick because they permit the penetration of a metal (i.e., are wettable by the metal) and thus, their functioning as a compliant layer has been partially compromised. Moreover, a requirement for specific mechanical properties in a ceramic may reduce the capacity to deliver desirable thermal properties. Further, ceramic articles which require the use of additional impregnation steps can be difficult to manufacture and add further cost to the construction of the composite body.
A need therefore exists to provide an inexpensive, reliable means for ensuring that ceramic articles will survive the stresses associated with metal casting so as to provide structurally sound ceramic-metal composite bodies. In particular, a need exists for ensuring that molten metal may be cast around a low strength ceramic article without degrading the mechanical properties of the ceramic and without degrading the mechanical properties of the composite body. In addition, a need exists to ensure that when molten metal is cast around a ceramic article and the thickness of the cooling metal is thin relative to the thickness of the ceramic article, and/or the tensile strength of the metal is low compared to the compressive strength of the ceramic, that the metal will not crack due to the development of tensile stresses therein.